selftests/tc-testing: add selftests for sfb qdisc

Test 3294: Create SFB with default setting
Test 430a: Create SFB with rehash setting
Test 3410: Create SFB with db setting
Test 49a0: Create SFB with limit setting
Test 1241: Create SFB with max setting
Test 3249: Create SFB with target setting
Test 30a9: Create SFB with increment setting
Test 239a: Create SFB with decrement setting
Test 9301: Create SFB with penalty_rate setting
Test 2a01: Create SFB with penalty_burst setting
Test 3209: Change SFB with rehash setting
Test 5447: Show SFB class

Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>